Pd-Ag-Ga DENTAL ALLOYS WITH INTERNAL OXIDATION CHARACTERISTICS

ABSTRACT

A Pd—Ag alloy system that utilizes internal oxidation by including a base metal addition of only Ga is provided. The Pd—Ag—Ga alloy is particularly well-suited for use in porcelain-fused-to-metal (PFM) applications because the alloy does not form a dark oxide upon heating. The high oxygen potential of the material ensures the development of a tenacious chemical bond with the porcelain when used in PFM applications. Moreover, the addition of gallium lowers the liquidus temperature of the alloy to improve castability and strengthens the alloy significantly.

FIELD OF THE INVENTION

This invention relates generally to improved palladium-silver alloys;and more particularly to a palladium-silver-gallium dental alloy withinternal oxidation characteristics.

BACKGROUND OF THE INVENTION

Compositional analyses of palladium-silver dental alloys intended forporcelain-fused-to-metal (PFM) applications reveal that all commercialalloys currently available utilize various combinations of base metals,such as, In, Sn, Ga, Zn and Mn to create materials which are easilycast, have high mechanical strengths, have suitable coefficients ofthermal expansion (CTE) and can create a proper bond with porcelains.However, all of these conventional alloys suffer from a seriouslimitation, namely, that they tend to develop dark oxides duringporcelain firing. These dark oxides are difficult to mask and cannegatively effect the overall esthetics of the porcelain, particularlyin the thin sections of the porcelain.

One method of addressing this aesthetic limitation is to usepalladium-silver alloys that do not include any base metals in theirformulations. While using such compositions resolves the problem of darkoxide formation, it introduces new limitations. Specifically, thesepalladium-silver alloys possess inferior mechanical properties, arepotentially prone to distortion during fabrication and in service, havehigher liquidus temperatures, and require an additional step in theirprocessing (the application of a bonding agent prior to the porcelainbakes) in order to develop a bond with the porcelain.

Accordingly, there is a need for a palladium-silver alloy that includesa base metal, possesses oxidation characteristics similar to those ofits base metal free counterparts, is strong enough to resist the forcesinvolved in dental articulation, can be easily cast, and improvesprocessing efficiency by not requiring a bonding agent application step.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a palladium-silver alloy suitable for PFMdental applications that prevents the formation of dark oxides through aprocess of internal oxidation of the alloy during the firing of theporcelain. According to a presently preferred embodiment, the Pd—Agalloy of the invention utilizes internal oxidation by including a basemetal of only gallium.

In another embodiment of the current invention, the Pd—Ag—Ga alloy mayfurther include small quantities of at least one grain refiner such asRu, Ir, and Re. In one such embodiment the grain refiner may have aconcentration of less than 3 wt %

In still another embodiment, the Pd—Ag—Ga alloy has a concentration ofGa of from 0.1 to 3 wt %. In another such embodiment the alloy has aconcentration of Pd of from 45 to 90 wt %. In yet another suchembodiment the alloy has a concentration of Ag of from 10 to 55 wt %.

In yet another embodiment, the alloy may also contain one or both of Auand Pt. In such an embodiment, the concentration of the Au or Pt (orcombination) may be up to 10 wt %.

In still yet another embodiment, the alloy has a composition ofPd_(51.2)Ag_(46.5)Ga_(1.5)Ir_(0.4)Ru_(0.4).

In still yet another embodiment, the invention is directed to a dentalappliance formed using the alloy of the current invention.

BRIEF DESCRIPTION OF THE DRAWING

The description will be more fully understood with reference to thefollowing figure, which is presented as an exemplary embodiment of theinvention and should not be construed as a complete recitation of thescope of the invention, wherein:

FIG. 1 provides a photographic image comparing the oxidationcharacteristics of an exemplary embodiment of a formed alloy inaccordance with the current invention and a base metal free, Pd—Agalloy.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is directed to a Pd—Ag—Ga alloy substantially freeof additional base metals. The current invention utilizes the concept ofinternal oxidation to prevent the formation of dark oxides whenporcelain is fired on the alloy such that the oxide color of the alloymatches that achieved when using palladium-silver alloys free of basemetals.

Internal oxidation is the process by which oxygen diffuses into an alloyand causes subsurface formation of oxides of one or more alloyingelements. The partial pressure of oxygen in a porcelain firing oven istypically at or below 0.2 atm (in air), and can be substantially highernear the metal/porcelain interface (>0.5 atm. For internal oxidationwithout external scaling, the oxygen diffusion into the alloy must begreater than the solute diffusion out of the alloy. The inventors of thepresent invention have discovered that it is possible to take advantageof this process of internal oxidation to produce an improvedpalladium-silver alloy for use in dental applications, and particularlyporcelain-fused-to-metal applications. It has been surprisinglydiscovered that the judicious use of gallium alone as the base metalalloying element instills internal oxidation characteristics to a Pd—Agalloy. The diffusion rate for such an alloy is well-suited for effectinginternal oxidation during the short thermal treatments involved inmultiple firing cycles and at the pressures used in porcelain firingovens (i.e., at or below 0.2 atm).

In accordance with the current invention, Pd may be provided in aconcentration range of from about 45 to 90 wt %, Ag may be provided in aconcentration range of from about 10 to 55 wt %, and Ga may be providedin a concentration range of from about 0.1 to 3 wt %. In addition tothese required materials, the alloy may also include optional materialsto improve some properties. For example, the alloy may includeconcentrations of other noble metals, such as, for example, Au or Pt ina concentration of up to 10 wt %. In addition, the alloy may optionallyinclude small quantities of grain refining elements, such as, forexample, Ru, Ir, and Re in a concentration of up to 3 wt %. Table 1,below provides a summary of the relative concentrations of elements,apart from contaminants that may be included in the alloys in accordancewith the current invention.

TABLE 1 Composition Ranges Concentration Range Element (wt %) Pd 45-90Au, Pt  0-10 Ag 10-55 Ga 0.1-3   Ru, Re, Ir 0-3

A specific example of a Pd—Ag—Ga alloy is provided in Table 2, below.For comparison purposes, a base metal free, Pd—Ag alloy (X0901A3) isincluded to highlight the similarity of the clean oxide color obtainableby both the prior art Pd—Ag alloy and the Pd—Ag—Ga alloy of the currentinvention. FIG. 1 provides a photographic comparison of the lack ofcoloration in both alloys following the porcelain firing cycles.(X0901A3 is on the left and X0901A7 on the right.)

TABLE 2 Exemplary Compositions Y.S. UTS EL Liquidus CTE Composition Wt %Alloys Condition (MPa) (MPa) % ° C. 25-600° C. Pd Ag Ir Ru Ga X0901A3980 C./15 min 237 391 29.7 1338.6 14.8 51.2 48 0.4 0.4 0 X0901A7 980C./15 min 309 504 28.7 1311 15.1 51.2 46.5 0.4 0.4 1.5

Beyond this aesthetic improvement, the ability of the Pd—Ag—Ga alloy inaccordance with the invention to internally oxidize leads to improvedphysical properties. For example, one of the advantages of using thePd—Ag—Ga alloy of the invention is that, because of the high oxygenpotential of the material, a clean or uncolored oxide forms under theporcelain, which ensures the development of a tenacious chemical bondwith the porcelain when used in PFM applications. Moreover, gallium (dueto its thermal and electronic properties) on a relatively smaller weightfraction basis also lowers the liquidus temperature to improvecastability and strengthens the alloy significantly. The data in Table 2demonstrates the improvements in thermal and mechanical properties ofthe alloys of the current invention, which again are of particularimportance when using the alloys for PFM applications.

One of the principal considerations in obtaining these improvedproperties is that the Pd—Ag alloy of the current invention must besubstantially free of other base metals. It will be understood that theterm “substantially free of other base metals” does not exclude thepresence of impurities and other minor additives having concentrationsof less than 1 wt % or less each.

In addition, it is important to note that prolonged heat treatment timesat high temperatures and under low pressures can force even those baseelements well-suited for internal oxidation up to the surface and forman external oxide on the bare metal surfaces. Accordingly, formingtemperatures, pressures, and treatment times must be controlled toensure that the partial pressure of oxygen during porcelain firingremains within the range such that oxygen diffusion into the alloy mustbe greater than the solute diffusion out of the alloy.

From the information above, it is evident that the Pd—Ag—Ga alloy of thecurrent invention displays significant improvement in physicalproperties while having the transparent oxide characteristics of thebase metal free alloys.

Although the above discussion has focused on the composition of thedental alloy, it should be understood that the current invention is alsodirected to dental appliances formed using the Pd—Ag—Ga dental alloy ofthe current invention. In particular, the alloy of the current inventionis well-suited for use in PFM applications where the dental appliance isformed by the firing of porcelain on a substrate formed of the alloy ofthe invention.

Doctrine of Equivalents

Those skilled in the art will appreciate that the foregoing examples anddescriptions of various preferred embodiments of the present inventionare merely illustrative of the invention as a whole, and that variationsin the overall composition of the present invention may be made withinthe spirit and scope of the invention. For example, it will be clear toone skilled in the art that the inclusion of additional non-basematerials and impurities within the indicated limits would not affectthe improved properties of the Pd—Ag—Ga alloy alloy of the currentinvention nor render the alloy unsuitable for its intended purpose.Accordingly, the present invention is not limited to the specificembodiments described herein but, rather, is defined by the scope of theappended claims.

1. A dental alloy capable of undergoing internal oxidation upon heating,consisting essentially of Pd, Ag and Ga, wherein the alloy issubstantially free of any additional base metal.
 2. The dental alloy ofclaim 1, having a composition of:Pd₄₅₋₉₀Ag₁₀₋₅₅Ga_(0.1-3)
 3. The dental alloy of claim 1, wherein thealloy further includes one or more additional base metals at aconcentration no greater than 1 wt %.
 4. The dental alloy of claim 1,wherein the alloy further includes at least one grain refiner.
 5. Thedental alloy of claim 4, wherein the at least one grain refiner isselected from the group consisting of Ru, Ir and Re.
 6. The dental alloyof claim 4, wherein the at least one grain refiner has a concentrationof from about 0 to 3 wt %.
 7. The dental alloy of claim 1, wherein thealloy further includes at least one additional noble metal selected fromthe group consisting of Au and Pt.
 8. The dental alloy of claim 7,wherein the additional noble metal has a concentration of up to 10 wt %.9. The dental alloy of claim 5, wherein the composition has a molecularformulation of Pd_(51.2)Ag_(46.5)Ga_(1.5)Ir_(0.4)RU_(0.4).
 10. A dentalappliance comprising a dental body formed at least partially from analloy material capable of undergoing internal oxidation upon heating,the alloy consisting essentially of Pd, Ag and Ga, wherein the alloy issubstantially free of any additional base metal.
 11. The dentalappliance of claim 10, wherein the alloy further includes one or morebase metals at a concentration no greater than 1 wt %.
 12. The dentalappliance of claim 10, wherein the alloy has the following composition:Pd₄₅₋₉₀Ag₁₀₋₅₅Ga_(0.1-3)
 13. The dental appliance of claim 10, whereinthe alloy further includes at least one grain refiner.
 14. The dentalappliance of claim 13, wherein the at least one grain refiner isselected from the group consisting of Ru, Ir and Re.
 15. The dentalappliance of claim 13, wherein the at least one grain refiner has aconcentration of from about 0 to 3 wt %.
 16. The dental appliance ofclaim 14, wherein the alloy has a molecular formulation ofPd_(51.2)Ag_(46.5)Ga_(1.5)Ir_(0.4)Ru_(0.4).
 17. The dental appliance ofclaim 10, wherein the alloy further includes at least one additionalnoble metal selected from the group consisting of Au and Pt.
 18. Thedental appliance of claim 17, wherein the additional noble metal has aconcentration of up to about 10 wt %.
 19. The dental appliance of claim10, wherein the appliance is a formed by fusing porcelain to the alloy.20. The dental applicant of claim 19, wherein the porcelain fusingoccurs in an oxygen containing environment, and where the fusingconditions are controlled such that the rate of diffusion of the oxygeninto the alloy is greater than the rate of diffusion of a solute out ofthe alloy.